Ink ribbon cartridges for printers are well known and rely on a substantially longitudinal advance of the ribbon in the course of a printing operation. Such longitudinal advance is uneconomical taking into account the unutilized area of the ribbon in the printing operation.
There have thus been developed ribbon cartridges with multistrike films that allow "overstrike" printing. This means that the ribbon is shifted approximately 1/5 of the width of a conventional character and then again printed on to achieve the conventional so called 5X over-strike ratio.
Because one side of the character has received fresh ribbon while the opposite side of the character has ribbon that has been overstruck five (5) times, the characters are darker on the side that has received the fresh ribbon. This is very apparent on characters that contain horizontal lines such as the letters "E" and "F" and on underscores. To avoid this problem it is known, to those skilled in the art, to vertically displace or dither the ribbon so that the horizontal strokes will be displaced on the ribbon and thus avoid striking the same region of the ribbon.
For the use of one-time film ribbons, which are necessary where very high quality printing is required, the vertical displacement per character must be larger than the height of the printed character itself or else large amounts of ribbon are wasted.
Until recently, displacing the ribbon vertically has involved external means either to lift ribbon or displace the entire cartridge. Such construction is noisy, costly, complex and requires a power drive system. A recent development (see the: above-mentioned U.S. Pat. No. 4,623,271) uses a roller containing an internal cam, mounted within the cartridge, to displace the ribbon vertically. Power for the vertical displacement comes from the rotation of the roller which is driven by the longitudinal advance of the ribbon through friction between the surface of the roller and the surface of the ribbon film.
While the cartridge described in the above-cited U.S. Patent has greatly improved the quality of overstrike printing, it has created a demand for even better cartridges. In particular, the described cartridge has several limitations. First, because only one side of the ribbon is lifted while the other side remains stationary and the print position is in the middle, the amount of vertical displacement at the print position is only one-half of the displacement at the roller. Second, there are limitations to the amount of lift that can be achieved. As the cam angle is increased, the force required to turn the roller also increases until the ribbon "slips" on the roller and the roller stalls. Third, it is not possible to add another lifting roller (with associated cam) to the side of the ribbon that is stationary because there is no way to synchronize both sides. Fourth, because the roller "stalls" when the pressure angle of the cam is too high, the amount of vertical displacement is limited and it is therefore impossible to achieve enough lift for the effective use of one-time film ribbon.